Cetaceans (baleen whales and toothed whales) are a distinctive group of mammals that has evolved unique adaptations to live in water. Their impressive fossil record traces their evolution from their terrestrial ancestors (Artiodactyla) to the present aquatic forms. All living cetaceans have flippers instead of limbs, possess a blowhole at the top of the head instead of an anteriorly positioned nose. They have also developed a characteristic head shape connected to their feeding mode: while baleen whales have lost their teeth and feed using keratinous plates called baleen, toothed whales have evolved the ability to locate their prey using sound waves (echolocation). Changes in prenatal development (ontogeny) played a major role in allowing these animals to evolve the skull adaptions they currently display related to their peculiar traits. Though, little information is available on their development, limiting our understanding of the role of ontogeny in the evolution of modern cetaceans. A better understanding of Cetacea life history can help inform conservation policies.
This project applied 3-dimensional (3D) imaging and cutting-edge geometric morphometric (GM) and phylogenetic methods to quantify the development of skull shape in a diverse sample of cetaceans in order to reconstruct how development has shaped the evolution and diversification of this unique group of mammals. I produced high-quality 3D datasets that will be shared in on-line repositories, making rare fetal specimens of toothed and baleen whales accessible for further research and for outreach initiatives and preserving them for future generations.
Specifically, I addressed the following questions: 1) How does skull prenatal development vary across toothed whales? Here, I focused on the changes in skull asymmetry, a peculiar trait of toothed whales, during ontogeny and how these differences influence the ecological and hearing adaptations in different species. 2) How does ear bones development differs between low-frequency hearing baleen whales and high-frequency hearing toothed whales? Here I used innovative methods to quantify changes in shape without the use of landmarks to study the differences in ossification and ontogeny in two major ear bones, the tympanic bulla and the periotic, in the two group of Cetacea, connecting them to the evolution of their disparate hearing adaptations. 3) How does skull development vary between toothed and baleen whales? Here I focused on changes in the rostrum and braincase during the ontogeny of the two groups to understand how ontogeny has influenced the evolution of their unique feeding adaptations and has helped them adapt to life in water.
We collected 3D data and built a dataset comprised of over 200 specimens of Cetacea at all stages of development, from embryos to adult, and spanning the diversity of the group. Using this impressive dataset and novel GM, statistical and phylogenetic comparative methods we were able to answer our main research questions. 1) We found that skull asymmetry progressively increases during ontogeny, and the level of asymmetry at the early fetal stages as well as the distribution of asymmetry in the adults is directly correlated with the evolutionary history of the species, while the high or low levels of asymmetry displayed in the adults are directly correlated with the ecology and hearing adaptations of the species. 2) Examining ear bone development, we found significant differences in ossification sequence and shape ontogeny in the periotic bone between the two groups, baleen and toothed whales, but not in the tympanic bulla. This is directly related to both their common ancestry and adaptations for underwater hearing, reflected in the tympanic bulla, and their different hearing frequencies, which depend on the internal morphology of the periotic bone. 3) We are currently finishing examining the relationship between changes in skull ontogeny and evolution of different feeding adaptations across this group. Generally, we see convergent but not shared ontogenetic patterns in braincase development between baleen and toothed whales, reflecting their convergent evolution towards a telescoped cranium, where the bones overlap and the nasals are retracted towards the back of the skull. The rostrum ontogeny is more disparate, reflecting major ecological differences particularly among toothed whales.
